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<h4 class="subsection" id="Defined-1"><span>4.2.3 Defined<a class="copiable-link" href="#Defined-1"> &para;</a></span></h4>

<a class="index-entry-id" id="index-defined"></a>
<p>The special operator <code class="code">defined</code> is used in &lsquo;<samp class="samp">#if</samp>&rsquo; and
&lsquo;<samp class="samp">#elif</samp>&rsquo; expressions to test whether a certain name is defined as a
macro.  <code class="code">defined <var class="var">name</var></code> and <code class="code">defined (<var class="var">name</var>)</code> are
both expressions whose value is 1 if <var class="var">name</var> is defined as a macro at
the current point in the program, and 0 otherwise.  Thus,  <code class="code">#if&nbsp;defined&nbsp;MACRO<!-- /@w --></code> is precisely equivalent to <code class="code">#ifdef&nbsp;MACRO<!-- /@w --></code>.
</p>
<p><code class="code">defined</code> is useful when you wish to test more than one macro for
existence at once.  For example,
</p>
<div class="example smallexample">
<pre class="example-preformatted">#if defined (__vax__) || defined (__ns16000__)
</pre></div>

<p>would succeed if either of the names <code class="code">__vax__</code> or
<code class="code">__ns16000__</code> is defined as a macro.
</p>
<p>Conditionals written like this:
</p>
<div class="example smallexample">
<pre class="example-preformatted">#if defined BUFSIZE &amp;&amp; BUFSIZE &gt;= 1024
</pre></div>

<p>can generally be simplified to just <code class="code">#if&nbsp;BUFSIZE&nbsp;&gt;=&nbsp;1024<!-- /@w --></code>,
since if <code class="code">BUFSIZE</code> is not defined, it will be interpreted as having
the value zero.
</p>
<p>If the <code class="code">defined</code> operator appears as a result of a macro expansion,
the C standard says the behavior is undefined.  GNU cpp treats it as a
genuine <code class="code">defined</code> operator and evaluates it normally.  It will warn
wherever your code uses this feature if you use the command-line option
<samp class="option">-Wpedantic</samp>, since other compilers may handle it differently.  The
warning is also enabled by <samp class="option">-Wextra</samp>, and can also be enabled
individually with <samp class="option">-Wexpansion-to-defined</samp>.
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